The neurotrophins, neurotrophic growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and neurotrophin 4/5 (NT-4/5) act via four receptors: the low affinity p75 neutrophic receptor (p75NTR), and the high affinity tyrosine kinase receptors; TrkA, TrkB, and TrkC. The low affinity receptor p75NTR binds and is activated by all four neurotrophins and has been reported to function independently from the other receptors. However, the Trk receptors are more selectively activated i.e. NGF is the selective ligand for TrkA, BDNF the ligand for TrkB and NT-3, 4/5 the ligands for TrkC. In addition it has been reported, when p75NTR and Trk proteins are co-expressed, they form complexes, which alter the signaling of both receptors (Huang and Reichardt, 2003). Indeed, it has been suggested that p75NTR facilitates the selectivity of each of the neutrophins for their respective Trk receptor.
The p75NTR is a member of the tumor necrosis factor receptor superfamily (TNFR-SF) and was the first member of this superfamily to be characterized fully. The superfamily (encoded by some 30 genes in humans) is defined by ligand-binding domains consisting of one or more (typically four) repeats of a 40 amino acid cysteine-rich domain (CRD) that was first identified in p75NTR (Johnson et al., 1986; Radeke et al., 1987). In contrast, no sequence motif is shared by the intracellular domains of all TNFR-SF family members. Consequently, signaling mechanisms of TNFR-SF proteins vary significantly.
An unusual feature of p75NTR structure is the existence of a disulfide-linked p75NTR dimer, formed via cysteinyl residues within the transmembrane domains. This disulfide linkage is required for effective neurotrophin-dependent signaling by p75NTR and plays an important role in the formation of an intracellular and extracellular domain (Vilar et al., 2009b). Neurotrophins exist physiologically as noncovalently associated dimers (Bothwell and Shooter, 1977) with a distribution half-life of approximately 5 min (Tria et al., 1994). Neurotrophin-dependent p75NTR activation involves association of a neurotrophin dimer with CRDs 2-4 of the two extracellular domains of a p75NTR dimer (He and Garcia, 2004). Recent studies support a model in which neurotrophin binding causes the two extracellular domains of p75NTR dimers to move closer together, forcing the intracellular domains to splay apart in a snail-tong-like motion centered on the disulfide bond and permitting association of the intracellular domains with the signaling adapter proteins, NRIF and TRAF6 (Vilar et al., 2009a, 2009b). Intra-transmembrane domain disulfide bonds, such as are present in p75NTR, have not been described previously in other TNFR-SF family members, or in any other membrane protein.
p75NTR undergoes sequential proteolytic cleavage by α-secretase and γ-secretase activities and matrix metalloproteinases (MMPs), releasing its intracellular domain (ICD) into the cytoplasm, in a manner analogous to the cleavage-dependent signaling pathway of Notch and β-amyloid precursor protein (Jung et al., 2003; Kanning et al., 2003). Cytoplasmic release of the p75NTR ICD by this pathway promotes signaling by associated NRIF (Kenchappa et al., 2006). The role of the extracellular domain of p75NTR, following the proteolytic cleavage by α-secretase and γ-secretase activities and MMPs isn't fully understood.
It has been documented that NGF and other neurotrophins (BDNF, NT-3 and NT-4/5) play a significant role in pathology for example pain due to osteoarthritis, pancreatitis, rheumatoid arthritis, psoriasis, pruritis and multiple sclerosis (Watanabe et al., 2010; Raychaudhuri et al., 2011; Barthel et al., 2009; Truzzi et al., 2011; McDonald et al., 2011; Yamaoka et al., 2007). It was been demonstrated that selective antibodies to any of the neutrophins; either NGF or BDNF, NT-3 and NT-4/5 significantly reduce pain. Furthermore, antibodies directed to the neurotrophin receptors p75NTR Trk A, Trk B or Trk C have also been demonstrated to be efficacious in models of pain (Orita S et al., 2010; Svensson P et al., 2010; Iwakura et al., 2010; Cirilio et al., 2010; Pezet et al., 2010; Hayashi et al., 2011; Chu et al., 2011; Ueda et al., 2010; Ghilardi et al., 2010; Fukui et al., 2010). Fukui et al., (2010) in a model of pain (mechanical allodynia following sciatic nerve crush) demonstrated significant efficacy on pain related endpoints following treatment with an anti-p75NTR antibody. It was concluded from this study that the treatment with a p75NTR inhibitory antibody reduced CGRP and p75NTR expression resulting in a significant reduction in pain.
The current invention demonstrates the extracellular domain of p75NTR, exogenous or endogenous following cleavage from the cell membrane, acts as a neurotrophin binding protein or soluble receptor to each of the neurotrophins NGF, BDNF, NT-3 and NT-4/5 and plays a significant role in the function, physiology and homeostasis of neurotrophin function. Moreover, we describe the use of the extracellular domain of p75NTR for modulating or neutralizing the pathological actions of the neurotrophic factors including NGF, BDNF, NT-3 and NT4/5, for example in models of static allodynia and thermal hyperalgesia. Thus the p75NTR neurotrophin binding protein finds use in the treatment of pain and other neurotrophic factor related pathologies such as psoriasis, eczema, rheumatoid arthritis, cystitis, endometriosis and osteoarthritis.